This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Mooring systems for marine vessels have not changed in principle since the early days of sail. A vessel is moored when it is fastened to a fixed object such as a bollard, pier, terminal, quay or the seabed, or to a floating object such as an anchor buoy. Mooring is often accomplished using thick ropes called mooring lines or hawsers. The lines may be constructed of fiber, natural or synthetic, or metal wire, or a combination of both wire and fiber. The lines are fixed to deck fittings on the vessel at one end, and fittings on the shore, such as bollards, rings, or cleats, on the other end. Once the lines are passed between the vessel and mooring hooks mounted on the pier or shore, they are heaved tight. On large ships, this tightening can be accomplished with the help of heavy machinery called mooring winches or capstans. For the heaviest cargo ships, more than a dozen mooring lines can be required.
As noted above, mooring lines are usually made out of synthetic materials such as nylon. Nylon has a property of being elastic. This elasticity has its advantages and disadvantages. The main advantage is that during an event, such as a high wind or the close passing of another ship, excess stress can be spread among several lines. On the other hand, if a highly-stressed nylon line does break, or part, it causes a very dangerous phenomenon called “snapback” which can cause fatal injuries.
Typical mooring systems work well when forces are relatively small and/or constant in force and direction. When the forces are varied, the vessel starts to sway and surge on its moorings. When the forces are large or their frequency approaches the natural frequency of the mooring system, the vessel can move enough to render brakes (i.e. exceed the capacity of the brakes on the mooring line winches on the vessel) or part the mooring lines.
Existing vessel-mooring systems manage these wave load issues in several ways:
Limiting the size of waves (and thus wave loads) that the vessel-mooring system can be exposed to. This is a minor issue for terminals in protected waters, such as harbors, but can have a significant effect on the ability to moor a vessel at an offshore terminal, such as the Adriatic LNG terminal.
Increasing the size and number of mooring lines. This makes the system stronger and able to work in larger waves, at a cost. There is still a wave limit, for example in long period waves, such as those longer than 10 seconds, ships can only be moored in wave heights of one meter or less. Most vessels are limited in the number of mooring lines they can handle.
Adjusting the stiffness of the mooring lines. Adding stretchable tails to the mooring lines changes the stiffness of the system, which in turn changes the natural period, which in turn changes the extent of resonance and dynamic amplification. At least one company has proposed the addition of mechanical springs. While these systems reduce the chance of a line breaking they can result in even more pronounced surge motions.
With the presently available technologies, vessel-mooring systems can reach their limits when the swell has a significant wave height of about one meter.
The need still exists for new approaches to the mooring of a vessel. In particular, there is a need for new approaches due to unsafe conditions created when wave heights increase beyond one meter, which is frequently encountered at offshore terminals.